1. FIELD OF THE INVENTION
The present invention relates generally to the field of communication, and more particularly to a computer bus having a wired-or function and operating in a differential transmission environment.
2. DESCRIPTION OF THE RELATED ART
Several new standards for high speed buses have been promulgated recently. Among these is the IEEE 1394-1995 High Speed Serial Bus standard, which shall be referred to herein as the 1394 standard. Similarly, a bus conforming to the 1394 standard will be referred to as a 1394 bus. Furthermore, as used herein, the term bus refers to any electrical signaling path between two or more devices.
The 1394 standard, often referred to by a commercial implementation of the standard known as FIREWIRE(trademark), defines a high speed serial bus with two different types of physical connection: backplane or cable. Cable connections are used for daisy chain configurations and are not of concern herein. The backplane configuration is useful in devices such as personal computers and other high-speed applications. The 1394 bus supports data transfer rates as high as 100 megabits per second.
The 1394 standard does not define the backplane environment (i.e., the interface technologies), but does require that a wired-or function be performed in whatever backplane environment is chosen. In this context, wired-or means that when several devices are connected to a common point (such as a bus conductor), the common point will be read as a logical xe2x80x9ctruexe2x80x9d by all devices when any one or more devices asserts a logical xe2x80x9ctruexe2x80x9d signal, and will be read as a logical xe2x80x9cfalsexe2x80x9d when no device asserts a logical xe2x80x9ctruexe2x80x9d signal. It should be noted that a logical true signal does not necessarily mean a higher voltage - many buses are implemented using active low, or reverse, conventions where a logical true signal is represented by a low voltage level (e.g., 0 volts) and a false is represented by a high voltage level (e.g. 5 volts). Thus, for example, a conventional AND gate performs an OR function if the inputs to the gate are defined as active low.
Given this wired-or requirement, the obvious choices for a 1394 backplane environment are single-ended transmission technologies with open collectors or open drains such as GTL (gunning transceiver logic) or BTL (backplane transceiver logic). However, single-ended open collector/drain technologies such as GTL/BTL cannot offer the noise immunity that differential transmission technology can. Unfortunately, differential transmission technology could not be used in a 1394 backplane because known differential transceivers do not perform the wired-or function required by the 1394 standard.
What is needed is a differential transmission circuit capable of performing a wired-or function.
The present invention overcomes the aforementioned limitations of the prior art by providing a method for, and a circuit capable of, performing a wired-or function in a differential transmission environment. The wired-or function is realized by enabling the differential driver portion of the differential transceiver when the signaling path is to be driven with a logical true signal and disabling the differential driver portion when a logical false signal is placed on the signaling path. In a preferred embodiment, a 1394 backplane bus node (a node is a single connection to the bus which may or may not be shared by multiple devices) is implemented using commercially available chips including a physical arbiter, a link controller, a differential transceiver, a programmable logic device, and some additional transistors.